Speech encoder

Abstract

A speech encoder is disclosed, in which, of the DCT coefficients after the discrete cosine transformation, a coefficient which has a large absolute value and exerts great influence on the tone quality is selected and encoded and zeros are inserted into the other unselected coefficients, so that selective encoding is carried out which does not seriously deteriorate the tone quality even when the coding rate is 8 kbps or below. In another arrangement, about three to 16 different selection patterns (vector patterns per frame) are used for the selective coding and a pattern which minimizes the coding error is selected and encoded to ensure optimum coding.

What I claim is: 
     
       1. A speech encoder of an adaptive transform coding system which adaptively encodes an analog speech signal through a block-by-block orthogonal transformation to frequency regions by a discrete cosine transform unit, comprising: a coefficient selector supplied with discrete cosine transform coefficients from said discrete cosine transform unit and selectively outputs a predetermined number of them in descending order of their absolute values for each frame; and   an encoder for encoding the output of said coefficient selector by adaptive transform coding.   
     
     
       2. A speech encoder of an adaptive transform coding system which adaptively encodes an analog speech signal through a block-by-block orthogonal transformation to frequency regions by a discrete cosine transform unit, comprising: at least three coefficient selectors supplied with discrete cosine transform coefficients from said discrete cosine transform unit and perform coefficient selection processing following different coefficient selection patterns for each frame;   a switch for selectively outputting any one of the outputs from said at least three coefficient selectors;   an optimum pattern selector for mutually comparing the outputs from said at least three coefficient selectors and the output of said discrete cosine transform unit to control said switch so as to select the output of said one of said coefficient selectors which outputs a vector pattern to minimize a quantizing error; and   an encoder for encoding the output of said switch by adaptive transform coding.